Analysis of flow effects in echo‐planar imaging

The effects on the phase of spins moving during echo‐planar imaging (EPI) acquisition were studied. Standard single‐shot and interleaved multishot blipped EPI acquisitions were considered, assuming either high gradient strength and slew rates or standard gradient strength and slew rates. A spiral k‐space trajectory was also considered. Flow components in the section‐select and phase‐ and frequency‐encoding directions were analyzed separately. While the effect of flow in the section‐select direction is identical to that in a standard two‐dimensional Fourier transform (2DFT) acquisition, flow in the phase‐ or frequency‐encoding directions can have substantial effects on the image, different from that in 2DFT imaging. The magnitude of these effects, which include displacement, distortion, and/or ghosting of vascular structures, is analyzed and predicted for a given velocity and direction of flow, the specific acquisition sequence, and the strength and slew rate of the gradients. For example, 50‐cm/sec flow along the phase‐encoding direction can cause a blurring of 1.25 cm full width at half maximum for blipped EPI with high‐strength gradients, assuming a 40‐cm field of view and 64 × 64 matrix.